Hydration layers at the graphite-water interface: Attraction or confinement

Water molecules at solid surfaces typically arrange in layers. The physical origin of the hydration layers is usually explained by two different reasons: (1) the attraction between the surface and water and (2) the water confinement due to the surface. While the attraction is specific to the particular solid, the confinement is a general property of surfaces; a differentiation between the two effects is, therefore, critical for research on interactions at aqueous interfaces. Here, we investigate the graphite-water interface, which is a widely used model system where the solid-water attraction is often considered to be negligible. Similar to previous studies, we observe hydration layers using three-dimensional atomic force microscopy at the graphite-water interface. We explain why the confinement could cause the formation of hydration layers even in the absence of attraction between surface and water by employing Monte Carlo simulations. Using additional molecular dynamics simulations, we continue to show that at ambient conditions, however, the confinement alone does not cause the formation of layers at the graphite-water interface. We thereby demonstrate that there is a significant graphite-water attraction.

Figure 1

A vertical slice of 3D AFM data showing the excitation frequency shift obtained at the graphite-water interface. The color scale ranges from $-7$ to 13 kHz.

Figure 2

MC simulation of a one-dimensional system of hard spheres with radius $\sigma$ between two hard walls spaced $40\sigma$ apart. In (a), the probability density $p$ of finding a sphere is plotted as function of the position $z$ between the two walls. The different curves show the probability densities for one to 15 spheres in the system. The space blocked by the walls is indicated by the shaded area in (b). In (c), (d), the space blocked by a single sphere placed far away and close to the wall, respectively, is indicated.

Figure 3

MC simulation of a one-dimensional system of spheres between two walls. The sphere-wall interaction is modeled with a purely repulsive hard-wall potential. The sphere-sphere interaction is modeled with an attractive interaction according to $-E_{\text{SS}}/d$, where $d$ is the distance between two spheres and $E_{\text{SS}}$ varies from zero (no attraction, black half-transparent line) to $10k_{\text{B}}T$ (red line).

Figure 4

Density profiles from MC simulations with attractive forces acting between the spheres and between the spheres and the wall. The sphere-wall interaction is modeled with a potential of $-E_{\text{SW}}/d$ that is superimposed on a hard-wall potential. The factor $E_{\text{SW}}$ varies from $k_{\text{B}}T$ (black half-transparent line) to $6k_{\text{B}}T$ (red line).

Figure 5

Comparison of MD simulation results (a) with the experimental AFM data (b). In (a), water-oxygen density profiles (normal to the surface, in $z$ direction) are shown for a force field with attraction (black curve) and for the purely repulsive carbon-water interaction (red curve). The vertical average over all experimental AFM data shown in Fig. 1 as function of the $z$-piezo displacement $z_{\text{p}}$ is shown in (b) for a direct comparison with the MD data. The offsets of the horizontal axes of (a) and (b) are arbitrarily aligned.

Figure 6

Setup of the MD simulation box used for the data presented in Fig. 5 as the black curve (the simulation with the full force field by Wu and Aluru [41]). The simulation frame corresponds to the equilibrated system.

Figure 7

Water-oxygen density profiles perpendicular to the surface for the Wu and Aluru [41] parametrization [black curve, same as in Fig. 5] and the parametrization from Werder et al. [52]. The results for the purely repulsive carbon-water interaction are shown in red [same as in Fig. 5].

Figure 8

Water-oxygen density profiles perpendicular to the surface for the purely repulsive carbon-water interaction and a decreasing amount of water.

Figure 9

Water-oxygen density profiles perpendicular to the surface for the purely repulsive carbon-water interaction and decreasing slab spacings of 6, 5, and 4 nm.